1. Field of the Invention
The present invention relates to a color filter which can reduce wavelength dispersion and can form a good circular polarization state over the entire visible light range and to a semi-transmissive semi-reflective liquid-crystal display device using such color filter, and also to a method for forming a phase difference (retardation) control layer and a method for manufacturing a color filter.
2. Description of the Related Art
A variety of liquid-crystal display devices have been developed, but according to a mode of using the light they can be generally classified into reflective liquid-crystal display devices that use an external light such as natural light or indoor light, transmissive liquid-crystal display devices that use an illumination light from a backlight, and semi-transmissive semi-reflective liquid-crystal display devices that can be both reflective and transmissive.
A device of the type shown in FIG. 4 has been suggested as one form of a semi-transmissive semi-reflective liquid-crystal display device. A semi-transmissive semi-reflective liquid-crystal display device 101a shown in FIG. 4 comprises an upper substrate 120 and a lower substrate 130 sandwiching a liquid-crystal layer 110.
A transparent electrode layer 135 and a semi-transmissive semi-reflective layer 134 are formed on the upper surface of the lower substrate 130. The transparent electrode layer 135 comprises indium tin oxide (ITO), and the semi-transmissive semi-reflective layer 134 comprises a reflective plate 134a made from a metal film such as aluminum and a light transmissive section 134b for transmission display.
Two phase difference plates 131, 132 are provided on the lower surface of the lower substrate 130, a polarization plate 133 is provided on the lower surface of the phase difference plate 132, and a backlight 140 is provided below the polarization plate 133. The reflective display using the external light is performed in a region (referred to hereinbelow as “reflective display region”) where the reflective plate 134a is provided, and the transmissive display using the illumination light of the backlight 140 is performed in the region (referred to hereinbelow as “transmissive display region”) where the transmissive section 134b is provided.
A black matrix 127 and a color layer 126 comprising a plurality of color pattern layers 126R, 126G, 126B are formed to a constant film thickness on the lower surface of the upper substrate 120, a protective layer 129 is provided on the lower surface of the color layer 126, and a transparent electrode layer 125 is provided on the lower surface of the protective film 129. On the other hand, similarly to the configuration formed on the lower substrate 130, two phase difference plates 121, 122 are provided on the upper surface of the upper substrate 120, and a polarization plate 123 is provided on the upper surface of the phase difference plate 122.
The phase difference plate provided on the lower surface of the lower substrate 130 comprises two phase difference plates 131 and 132, and the phase difference plate provided on the upper surface of the upper substrate 120 likewise comprises two phase difference plates 121 and 122. Using the two phase difference plates in such manner enables them to function as quarter-wavelength phase difference plates of a broad band type and to convert a linearly polarized light into a circularly polarized light over almost the entire visible light range.
Furthermore, because the configuration is employed in which the polarization plate and phase difference plate are provided respectively on the upper surface of the upper substrate 120 and the lower surface of the lower substrate 130, both the incident light from the backlight 140 and the incident light from the outside can be converted into the circularly polarized light.
However, the drawback of the above-described conventional liquid-crystal display device 101a in which two phase difference plates are provided via adhesive layers between the upper and lower substrates 120, 130 and the polarization plates is that because the thickness of the entire device is rather large, such a configuration cannot be adapted to decrease the device thickness.
Furthermore, the mixed light illuminated from the backlight 140 is transmitted via the polarization plate 133, becomes a linearly polarized light with the predetermined angle, and then passes through the quarter-wavelength phase difference plates 132, 131 and becomes a circularly polarized light, but if the circularly polarized light is reflected at the rear surface of the reflective plate 134a, the orientation of the circularly polarized light is inverted. For this reason, this reflected light is further polarized by the quarter-wavelength phase difference plates 131, 132, becomes a linearly polarized light with a transmission axis perpendicular to that of the aforementioned linearly polarized light, and falls on the polarization plate 133. As a result, this light is absorbed in the polarization plate 133, rather than transmitted therethrough. The resultant problem is that the collected light cannot be reflected again toward the liquid-crystal layer 110 by the backlight reflection plate 141 and the illumination light cannot be reused.
Japanese Patent Application Laid-open No. 2004-004494 discloses a semi-transmissive semi-reflective liquid crystal display 101b in which the phase difference control layer 137 overlaps a reflective display region in the lower substrate 130 and is finely patterned with the object of resolving the above-described problem. As shown in FIG. 5, with the liquid crystal display 101b of Japanese Patent Application Laid-open No. 2004-004494, a structure is obtained in which it is not necessary to provide two phase difference plates on the upper surface of the upper substrate 120 and lower surface of the lower substrate 130 and the light of the backlight can be reused.
Furthermore, in the liquid crystal display 101b, the incident light that is reflected by the reflective plate 134a passes twice through the color layer 126 and the resultant effect is adjusted by making the thickness and color density of the color layer 126 in the reflective display region less than those of the color layer in the transmissive display region.
One of the means for finely patterning the phase difference control layer 137 is a formation method by which a UV-curable liquid-crystal material is used, the liquid-crystal material is set to a constant orientation state, then the liquid-crystal material is locally photopolymerized by using a photolithography method, the phase difference control layer 137 is pattered, and the regions other than the phase difference control layer 137 are removed by etching.
Furthermore, an invention has been suggested that relates to a semi-transmissive semi-reflective liquid-crystal display device 101c of a structure in which the color layer 126 of the liquid-crystal display device 101b shown in FIG. 5 is provided on the lower surface of the upper substrate 120 and the phase difference control layer 137 is provided on the lower surface of the color layer 126, and an invention relating to a color filter 102 in such liquid-crystal display device 101c has also been suggested.
The liquid-crystal display device 101c is shown in FIG. 6. As shown in the figure, the advantage of the liquid-crystal display 101c is also in that it is not necessary to provide two phase difference plates on the upper and lower substrates 120, 130 and that the illumination light of the backlight 140 can be reused.
However, in the conventional semi-transmissive semi-reflective liquid-crystal display devices 101b, 101c shown in FIGS. 5 and 6, the color of light transmitted through color pattern layers constituting the color layer 126, that is, the wavelength of the visible light beam differs between the color pattern layers through which the light passes. As a result, in the phase difference control layer 137 in the conventional device having only the function of providing a uniform wavelength shift to the incident light, an optimum phase difference amount cannot be generated for each transmitted light. In other words, in the conventional phase difference control layer 137, good circularly polarized light cannot be obtained over the entire visible light range and a transition to a broader band in a liquid-crystal display device cannot be performed.
In order to broaden the band, a phase difference film having an inverted wavelength dispersion characteristic (for example, WRF film series manufactured by Teijin Corp.) or a phase difference film in which a half-wavelength phase difference plate is combined with a quarter-wavelength phase difference plate is generally used. However, polymerizable liquid-crystal materials generally have a wavelength dispersion characteristic of a refractive index such that the refractive index anisotropy increases at a short wavelength, and when the films of the same thickness are produced from the same material, a good circularly polarized state cannot be formed over the entire wavelength range of visible light.
This issue is described below in greater detail. The phase difference amount required for a phase difference plate (and the phase difference control layer) is different for each color (actually, the wavelength of the transmitted light) of the color pattern layer constituting the color layer. For example, if the central wavelength of the red color light is taken as 650 nm, the central wavelength of the green color light is taken as 550 nm, and the central wavelength of the blue color light is taken as 450 nm, then the phase difference amount required for a quarter-wavelength phase difference plate will be 650/4=163 nm for the red color light, 550/4=138 nm for the green color light, and 450/4=113 nm for the blue color light. For this reason, when optical designing is performed, the phase difference plate is designed so as to realize a circularly polarized light with a wavelength close to the green color (close to 550 nm) where the visual sensitivity is the highest.
Therefore, in the red color region and blue color region, the phase difference amount is insufficient or excessive, and a perfect circular polarization cannot be obtained. The resultant problem is that when black display is performed on a liquid-crystal screen, light components leak in those regions and a black display with a violet tint is obtained.
The present invention was created to resolve the above-described problems and it is an object of the present invention to provide a color filter for realizing a semi-transmissive semi-reflective liquid-crystal display device that can be reduced in thickness, this color filter making it possible to reduce the wavelength dispersion and to form a good circular polarization state over the entire visual light range.
Another object of the present invention is to provide a semi-transmissive semi-reflective liquid-crystal display device configured by using the color filter in which light leak during black display is small and high-grade display can be realized.
Yet another object of the present invention is to provide a method for forming a phase difference control layer for realizing the aforementioned color filter and liquid-crystal display device.
Still another object of the present invention is to provide a method for manufacturing a color filter suitable for the above-described liquid-crystal display device.